1. Field of the Invention
This invention relates to an adaptive predistortion circuit with memory for a digital transmission system transmitting input data suitable to levels of a signal constellation C, and occur at the rate of a symbol clock having a period T with the aid of a modulator and a power amplifier that distorts the signal by creating smeared spots (clouds) in the constellation, the circuit comprising:
a predistortion circuit for predistorting in the reverse sense the in-phase and quadrature input data symbols a.sub.n =(a'.sub.n, a".sub.n) prior to their entering the amplifier in order to transmit expected levels, the predistortion circuit simultaneously taking into account L received input data symbols a.sub.n+(L-1)/2 . . . a.sub.n . . . a.sub.n-(L-1)/2 which are temporarily stored in a set of input shift registers,
an adaptation circuit for continuously adapting the predistortion circuit to the stream of input data symbols in response to a demodulation of the stream of transmitted data symbols, by means of a comparing circuit that compares the in-phase and quadrature input data symbols (a'.sub.n, a".sub.n) with the in-phase and quadrature demodulated data symbols (y'.sub.n, y".sub.n) and by means of an interface circuit that realizes an adaptation algorithm.
The invention finds its applications in digital transmission systems such as data transmission modems, microwave radio relay links, and space communication systems.
2. Description of the Prior Art
For an efficient use of the available spectrum, the current digital transmission systems, specifically the microwave radio relay links and the systems for transmitting data over the telephone channels, use modulation methods having a large number of phase and amplitude states. However, these modulation methods are very sensitive to all types of distortion and, of course, to non-linear types of distortion caused by amplifiers, mixers and other non-linear circuits in the transmission chain. A particularly critical point with respect to the microwave radio relay links and satellite transmission is the non-linearity of the transmitting power amplifier or the on-board power amplifier in the case of satellite transmissions. These amplifiers are known for their non-linear characteristics. If they are used in their linear zone, the full extent of their power is not utilized. If they are made to operate near to their saturation power level, they will unacceptably distort the signal. In practice, for a given power amplifier, one fixes the level of the transmitted signal in order to establish a compromise between the signal-to-noise ratio and the non-linear distortion undergone by the signal. Thus, the optimum operating point of the amplifier is the one at which the joint effects of the additive noise of the channel and the non-linear distortion of the amplifier are minimized. For modulation methods with a large number of states (64-QAM and 256-QAM, for example) this point is remote from the saturation power level of the amplifier, which implies that the latter is not used efficiently. In order to enhance its efficiency, predistortion techniques (fixed or adaptive) are currently used which make it possible to reduce the effect of the power amplifier's non-linearity on the transmitted signal.
A currently used predistortion technique consists of inserting in the intermediate-frequency stage of the transmitter a non-linear circuit providing an approximation of the inverse function of the power amplifier whose non-linearities one seeks to compensate. If the exact inverse of the function of the amplifier could be synthesized, this technique would make it possible to have a perfect signal at the output (without any non-linear distortion). However, this cannot be realized because the exact inverse function would require a circuit of infinite complexity. In practice one is satisfied with making an approximation and in most cases the Taylor series representing the non-linear function of the amplifier is limited to the third order and a predistortion circuit is synthesized, also of the third order, in a manner such that the two cascaded circuits no longer have third-order distortion. Higher-order terms (fifth order and seventh order) appear at the output but have smaller amplitudes compared to the initial third-order distortion. The result is then a certain improvement of the performance of the system. A disadvantage of these predistortion circuits in the intermediate frequency stage resides in the fact that they are analog circuits. They are hard to make adaptive and require readjustment from time to time in order to compensate for the variations of the amplifier response according to time and temperature. This predistortion technique has to be dispensed with if one wishes to have an automatic transmit power control.
Another more recent predistortion technique consists of modifying the alphabet of the data to be transmitted. This technique, called "Data Predistortion", or "Baseband Predistortion" is known from U.S. Pat. No. 4,291,277 and from the article by A. A. M. SALEH and J. SALZ "Adaptive Linearization of Power Amplifiers in Digital Radio Systems", Bell System Technical Journal, Vol. 62, April 1983, pp. 1019-1033.
In the article by A. A. M. SALEH and J. SALZ, FIG. 1 presents a schematic representation of an adaptive predistortion circuit which supplies to the input of the modulator a distorted constellation on the basis of the original "square" constellation, for example, an amplitude modulation of two quadrature carriers (QAM). The amplifier acts on the constellation by producing a net compression and a net rotation of the points having large amplitudes. In order to compensate for this effect the original constellation is distorted so that it resumes its original square shape after passing through the power amplifier. Thus, when the distortion circuit is optimized, it forms the inverse of the power amplifier (apart from a constant gain and a constant phase shift) and allows for perfect compensation of the non-linearities of the amplifier. In order to make this circuit adaptive, the signal is recaptured at the output of the amplifier, demodulated, then sampled at the symbol transmission rate 1/T and these samples are compared to the points corresponding with the QAM constellation used. These comparisons make it possible to obtain a control signal which enables optimization of the predistortion circuit with the aid of a conventional algorithm. However, the scheme used in FIG. 1 is very simplistic because it does not have any filtering before the modulator or before the power amplifier. Thus, it does not correspond with the solution generally used. In effect, in the real systems a spectral shaping signal filtering of the Nyquist type is always used which makes it possible to limit the signal band while quaranteeing a zero intersymbol interference at the decision instants. This filtering is generally equally divided between the transmit and receive ends so as to maximize the signal-to-noise ratio at the decision instants. In such systems the effect of the non-linearity of the amplifier is twofold: the constellation is not only deformed but intersymbol interference appears, associating a cloud of points to each point of the constellation. With the above-described predistortion technique it is, however, not possible to compensate for this second effect.
U.S. Pat. No. 4,291,277 also describes an adaptive predistortion technique. The objects aimed at are similar. In order to perform the correction the symbol a.sub.n received at a given instant is used. The Patent also indicates that it is possible to associate other symbols received at different instants by storing them in an input register. However, this approach is much more complex to implement by way of experiment and U.S. Pat. No. 4,291,277 is restricted to dealing with the case of a single symbol, without discussing this difficulty. In effect, when various symbols are taken into account received at instants (earlier and/or later) different from the instant at which the symbol a.sub.n is received, it is necessary to take account of a large number of data symbols for modifying the process which is to take place. In addition, in this circuit, the predistortion circuit is adapted stochastically, that is to say, that the error signal is directly used for correcting the predistorted symbols. This is insufficient if a filtering precedes the amplifier because the points of the received constellation are then clouds of points. Finally, it should be observed that the filtering appearing before the modulator in U.S. Pat. No. 4,291,277 is not a filtering performing a transmit signal shaping, but only a filtering intended to eliminate the high frequencies caused by the digital-to-analog conversion and does not make it possible to limit the transmit signal band to a value below 1/T.